chapt09 lecture

Lecture Outlines

Physical Geology, 12/e
Plummer & Carlson
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


Mass Wasting
Physical Geology 12/e, Chapter 9


Mass Wasting
• Mass wasting is downhill movement of
masses of bedrock, rock debris or soil,
driven by the pull of gravity
• Landslides have been far more costly
in the U.S., in terms of both lives and
dollars, than all other geologic and
weather hazards combined
• Mass wasting is, with proper planning,
perhaps the most easily avoidable of all
major geologic hazards


Classification of Mass Wasting
• Types of mass wasting are
classified based on:
– Rate of movement (speed)
• Wide range from < 1cm/year to >100
km/hour

– Type of material
• Did moving mass start out as solid
bedrock or as debris (unconsolidated
material at Earth’s surface)

– Movement Type
• Flow, slide, or fall


Classification of Mass Wasting
• Types of movement
– Flow
• Descending mass moves downhill

as a viscous fluid

– Slide
• Descending mass remains
relatively intact, and
descends
along well-defined surfaces
• Translational slide - movement
to motion

along plane parallel

• Rotational slide (slump) - movement
surface

along a curved

– Fall
• Material free-falls or bounces down a cliff


Mass Wasting Chart


• List the three classifications for which mass
wasting is determined. (Not the three types
of movements.)


Some types of Mass Wasting
Page 222
What are the variables? How is the data chart
set up?
How does the chart specify material?


Tell the specific type of mass
wasting:
1. flow, slow velocity, soil =
creep
2. bedrock, fall, fastest =
rockfall
3. debris, flow, varying speeds =
debris flow
4. bedrock, slide, varying speeds =
rockslide

Factors Controlling Mass Wasting
page 226-228
explain each one


Factors Controlling Mass Wasting
• Factors making mass wasting likely:
– Steep slopes
• Shear forces maximized by gravity

– Large relief
• (large elevation change from top of
mountains/hills to valley floor)

– Thick layer(s) of loose rock,
debris, soil
– Presence of water
• Lubricates moving rocks/debris/soil

– Lack of vegetation
• No roots to hold rock/soil in place

– Seismic (earthquake) activity


Factors Controlling Mass Wasting

Common Types of Mass Wasting
• Creep (or soil creep)
– Very slow downslope movement of
soil
– Major contributing factors include
water in soil and daily freeze-thaw
cycles
– Can be costly to maintain homes, etc.,
on creeping ground as foundations,
walls, pipes and driveways crack and
shift downslope over time


Common Types of Mass Wasting
• Debris flow - mass wasting in which motion takes
place throughout the moving mass (flow)
– Earthflow - debris moves downslope, slowly or rapidly, as a
viscous fluid
• Commonly occurs on steep hills, with thick debris cover, after
heavy rains
• Solifluction is an example

– Mudflow - flowing mixture of debris and water, usually down a
channel
• Most likely to occur on steep unvegetated slopes with thick debris
cover
• Heavy rains on the slopes of stratocone volcanoes with fresh ash
layers often triggers

– Debris avalanches are very rapid and turbulent

• Can reach speeds of several hundred km/hr


Types of Mass Wasting
• Rockfall - when a block of bedrock breaks free and
falls or bounces down a cliff
– Commonly an apron of fallen rock fragments (talus)
accumulates at cliff base

• Rockslide - the rapid sliding of a mass of bedrock
along an inclined surface of weakness

• Rock avalanche - a very rapidly moving, turbulent
mass of broken-up bedrock

• Debris slide - a coherent mass of debris moving
along a well-defined surface

• Debris fall - a free-falling mass of debris


Preventing Landslides
page 238 – 243
explain how to prevent mass wasting


Preventing Landslides
• Preventing mass wasting of debris
– Construct retaining wall with drains
– Don’t oversteepen slopes during construction

• Preventing rockfalls and rockslides on
highways
– Remove all rock that is prone to sliding
– “Stitch” together outcrop
• Important to know the susceptibility of land to mass
wasting before building any road or structure!



End of Chapter 9